The present invention relates to hydraulic devices for industrial vehicles, and, more particularly, to hydraulic devices preferably used in battery type forklifts.
Generally, a battery type forklift has a motor for driving wheels. The forklift further includes a power steering device and a carriage device that are operated through hydraulic pressure. In other words, the power steering device and the carriage device are supplied with hydraulic fluid from a hydraulic pump. The hydraulic pump is activated by a motor. The motor for driving the wheels and the motor for activating the hydraulic pump are powered by a battery, which is installed in the interior of a body frame of the forklift.
FIG. 7 shows a hydraulic device for a battery type forklift that is described in Japanese Unexamined Utility Model Publication No. 56-84600. The hydraulic device has a pair of hydraulic pumps 81, 82 that supply hydraulic fluid to a cylinder 80 of a carriage device. The hydraulic pump 81 is activated by a motor 83, and the hydraulic pump 82 is activated by a motor 84. When only the pump 81 is activated, the cylinder 80 is operated at a relatively low speed. When both pumps 81, 82 are activated, the cylinder 80 is operated at a relatively high speed.
In addition to the cylinder 80, the hydraulic pumps 81, 82 supply hydraulic fluid to a power steering device (not shown) through a throttle valve 95. If the power steering device is operated together with the cylinder 80, the hydraulic pump 81 must constantly supply hydraulic fluid to the power steering device and the cylinder 80, regardless of the speed at which the cylinder 80 is operated. The displacement of the pump 81 must thus be relatively large. Also, the motor 83, which activates the pump 81, is formed relatively large to increase the output of the motor 83. The motor 83 thus consumes a relatively large power supplied from a battery. Further, even when only the power steering device is operated, the motor 83 consumes the battery power by driving the pump 81. This shortens durability of the battery.
FIG. 8 shows a hydraulic device described in Japanese Unexamined Patent Publication No. 56-23200. The hydraulic device has a tandem pump that includes a carriage pump 85 and a power steering pump 86. The carriage pump 85 and the power steering pump 86 are driven by a motor 87. The motor 87 is controlled by a controller 88.
A pair of tilt cylinders 90 and a lift cylinder 89 actuate a carriage device. The tilt cylinders 90 are connected to the carriage pump 85 through a tilt valve 97. The lift cylinder 89 is connected to the carriage pump 85 through a lift valve 98. A power steering cylinder 91 is connected to the power steering pump 86 through a steering control valve 99.
Since the motor 87 must drive the carriage pump 85 and the power steering pump 86, the motor 87 is formed relatively large to increase the output of the motor 87. This increases power consumption of the motor 87.
Accordingly, it is an objective of the present invention to provide a hydraulic device for industrial vehicles that lowers power consumption while improving carriage performance.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, the invention provides a hydraulic device for an industrial vehicle that operates a carriage device and a traveling device by supplying the carriage device and the traveling device with hydraulic fluid discharged by a hydraulic pump driven by a motor.
The hydraulic device includes a first hydraulic pump for supplying the hydraulic fluid to the traveling device, a second hydraulic pump for supplying the hydraulic fluid to the carriage device, wherein the second hydraulic pump and the first hydraulic pump together form a tandem pump, a first motor for driving the first hydraulic pump and the second hydraulic pump, a third hydraulic pump for supplying the hydraulic fluid to the carriage device, wherein the third hydraulic pump is operated in accordance with an operation amount of the carriage device, and a second motor for driving the third hydraulic pump.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.